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Patented Mar. 30, 1943 UNITED STATES CAVITY RESONATOR Herbert Buchholz, Berlin, Germany, assigner to General Electric Company, a corporation oi' New York Application July 12, 1940, Serial No. 345,038 In Germany September 5, i939 1v Claim.

It is known in the artthatoscillatory structures or systems employing coils and condensers are not practicable in ultra short wave work. Hence, for the various purposes in which such oscillatory structures are required it has been customary to use'either Lecher Wire systems or coaxial lines of certain length, so-called )./4 conto secure the desired resonance eilects, the losses incurred in such schemes prove still unduly high in certain practical cases.

lStarting from purely theoretical considerations, it has been discovered that resonance effects are obtained in a completely closed non-magnetic metal body or enclosure in which an exciting system is mounted in a way quite similar to that known from Lecher wire systems. The present invention is predicated upon this consideration. It consists in using hollow or cavity or chamber bodies for oscillatory structures (resonators) in ultra short wave work.

The novel resonators created for the said purpose distinguish themselves from oscillatory structures of the kind known in the prior art, especially becauseof the'A fact that'their construction is extremely simple, for they involve and require bodies of the simplest geometrical form.

' However, one essential advantage is that they involve considerably smaller losses lthan the oscill 3o quencies in the pre nce of which a state of reslatory structures heretofore used.

Chamber resonators of the kind here disclosed (sometimes called cavity resonators), in line with what follows from general considerations, have the property of possessing an infinite series of exactly fixed and defined resonance frequencies, starting from a certain minimum frequency which is governed by the dimensions of the metallic body or chamber. The nature and the mounting of the exciting system in the interior of the chamber or hollow body or cavity has no influence upon the critical frequencies. To excite the resonance wave, which may be an electrical or magnetic transversal wave or else a corresponding longitudinal wave, recourse is had to suitable exciter systems of the kind known from the ultra short 45 wave art. For example, for exciting electric transversal waves a simple circular current loop is employed. To excite a magnetic wave in the form of a resonance wave, there may be used an 60 electrical dipole suitably disposed in the linterior of the resonator or an exciter system of an equivalent nature. An exciter system of the kind here concerned could be formed, for instance, of a rim or circle of wire loop umts connected in parallel so disposed that theyresult in a sort of annular or toroidal coil.

In what follows a number of exemplified embodiments of such chamber resonators shall be described by reference-to the drawing. However,

While itis true that it is readily feasible x by no means restricted to the forms ofconstruction here shown by way of example. In fact, it will be readily understood that there is a multiplicity of similar simple chambers with which 5 very definite resonance effects are obtainable.

Fig. 1 is a top plan view and Fig. la a side view of a cylindrical cavity resonator of the invention; Fig. 2 is a perspective view and Fig. 2a a side view of another cavity resonator in accordance with l0 the invention; Fig. 3 is a perspective-view of a prism cavity resonator of the invention; Fig, 4 is an equivalent electrical circuit diagram of the three resonators of Figs. 1, 2 and 3; Fig. 5 shows how a cavity resonator ofthe invention can be excited and the output taken therefrom; and Fig.

5a shows the equivalent electrical circuit of a system as shown in Fig. 5.

To cite one example, the assumption shall be made that the metallic body has' the shape of a 4cylinder R (Fig. l) having a radius a and a length 2 (note Fig. 1a). Inside the cylinder is an exciting system E in the form of a simple circular' loop, the current supply leads St of which are indicated. By means of such an exciter system electrical waves are generated. The dielectric of this form of construction (as Well as in those hereinafter to be discussed) is air (with e=1). As already pointed out above,. such a chamber resonator has an infinite number of critical freonance arises. In ,e instance here under con- 1 21rd. critica 2* A?) +Je Where n equals integral parameters (l, 2, 3,

4 .and im the roots of the equation. 40 J1 (z) =0 (cylindrical function of the first kind or Bessel function). In case of magnetic excitation, im must beput in lieu of the square root term 11,- the former representing-the roots of equation Jo(.'c) =0. i

This equation will result in the lowest resonance frequency by choosing the lowest parameter (11:1) and using for im also the lowest value. The lowest value of 51, which may be found in tabulations are:

' chamber with a concentric hole; that is to say. v

by using a hollow cylinder for the resonator it is expressly-emphasized that the invention is (Figs. 2 and 2a). In this form of construction making:

the critical frequencies of the resonator turn out to be somewhat higher throughout, although the diierenc as compared with the simple cylindri-1 cal i'orm are only slight. In the presence ot a radius ration of 11:11:10, the lowermost limiting frequency is at the most from ilve to ten percent higher than in the rst embodiment.

In a further embodiment the resonator is ot prismatic form, the edges of which have dimensions 2a, 2b and 2c (Fig. 3). The excitation shall be assumed to be the same as in the first exempiiiled embodiment bythe aid of a loop. The critical frequencies are here -calculable on the basis oi' equation:

critical- Ws 1/ a +(a) +(2c) A where m, n, p, again integral parameters, .have these values:

mmm) =0, l, 2, 3

The lowermost critical frequency is obtained by It is quite readily feasible to suitably dispose inside the metallic chamber or body also a plurality ot electrical or magnetic exciter systems. so that the chamber resonator could, for instance, be used as a selective transformer or coil. An embodiment 'of this kind is4 illustrated in Fig. 5. Two electric exciting systems in the form of two current loops could be mounted inside the resonator chamber in suchv a way that the resonator acts like a transformer in line with the equivalent circuit diagram in Fig. 5a.

` The above formulae go to show that the main iield of application of the resonators lies within the decimeter wave band, for thereason that the geometric dimensions of the resonators would become unduly large if theywere to be used for longer waves. However, theoretically speaking, no limitation is imposed, that is, the invention is not restricted tothe said field.

What is claimed is:

An ultra short wave oscillatory circuit comprising a hollow cylindrical body having metallic end plates, a tubular conductor located along the axis of said cylindrical body and extending through said end plates, and a circular loop in the interior of said body and aroundsaid tubular conductor, the terminals of' said loop bengadjacent each other and entering the interior of said tubular conductor for coupling to external translation apparatus.

- HERBERT BUCHHOLZ. 

